Conventional methods for solvent processing to form Group I base stocks can produce various types of high viscosity base stocks, such as bright stocks. However, solvent processing is generally less effective at reducing the sulfur and/or nitrogen content of a feed, which can result in base stocks with detrimental amounts of heteroatom content. Hydrotreating and/or hydrocracking processes can be used prior to and/or after solvent processing for heteroatom removal, but such hydroprocessing can significantly reduce the viscosity of the resulting hydrotreated base stock.
More generally, high viscosity base stock capacity has declined as refiners have transitioned from solvent processing for lubricant base stock production to catalytic processing. While catalytic processing is suitable for making lower viscosity base stocks, the hydrotreating and hydrocracking processes used during catalytic processing tend to limit the ability to make base stocks with viscosities greater than about 10 cSt at 100° C.
Other options for high viscosity base stocks can include specialty polymeric materials, such as the poly-alpha-olefins in ExxonMobil SpectraSyn™ base stocks. Such polymeric base stocks can have bright stock type viscosities with reduced or minimized sulfur contents. However, production of such polymeric base stocks can be costly due to a need for specialized feeds to form the desired polymer.
U.S. Pat. No. 4,931,197 describes copolymers formed from α,β-unsaturated dicarboxylic acid esters and α-olefins. The copolymers are produced by copolymerization in the presence of a peroxide catalyst at temperatures of 80° C.-210° C. The copolymers are described as suitable for use as a lubricant for the shaping treatment of thermoplastic plastics.
U.S. Pat. No. 4,913,794 describes a process configuration for producing high viscosity lubricating oils. A lubricating oil and an organic peroxide are co-injected into a reactor to form a higher molecular weight product. The Examples provided in U.S. Pat. No. 4,913,794 describe processes using 10 wt % of an organic peroxide in the co-injected feed.